1. Field of the Invention
This invention relates to a vacuum suction device for holding, by suction, parts to be ground (workpieces) such as a semiconductor wafer and a glass plate on the face of a suction plate.
2. Description of the Prior Art
When precision grinding is done or accurate measurement is made on silicon wafers for integrated circuits, glass plates for liquid crystal display devices, or other parts, they are held on a clamp by suction. This type of clamping methods include those for holding parts by suction on a metal suction plate having grooves or holes therein (not shown) and for holding parts by suction on a suction plate formed of porous material (FIGS. 1 and 2).
However, for example, when grinding a silicon wafer, the method in which some roughness such as grooves and holes is formed in a metal suction face will cause a gap between the suction face and the silicon wafer, when loaded. As a result, the wafer is locally deformed, and at the time of cutting operation, the wafer's dimensions are fouled up or its surface becomes uneven. For example, when grinding a wafer with a thickness of 0.1 mm to 0.5 mm, depressing the wafer from the above causes it to curve downward in the grooves, and if grinding is done as it is, the portions of the wafer positioned in the grooves will swell out and appear on the surface when the depression is released. This cannot be neglected for wafer grinding which requires a high degree of flatness, for example, within .+-.5 .mu.m.
On the other hand, the method using a porous material as a suction plate is, for example, shown in FIGS. 1 and 2. FIG. 1 shows the case where suction grooves 15, 16 and 17 are provided in a portion of a base 14 abutting on the bottom of a suction plate 11. The suction plate 11 is formed of porous material surrounded around its outer periphery by an airtight material such as metal (a part of the base 14). On the upper surface 11a of the suction plate 11, a workpiece 10 such as a wafer is held by suction. However, if a wafer is ground with the vacuum suction device thus constructed, grinding fluid gets into the vacuum suction device from the portion of the suction plate 11 which is not covered with the wafer 10 through the suction grooves 15, 16 and 17 and a pipe 18, thereby lowering the degree of vacuum and affecting the stability of suction force. Then, referring to FIG. 2, there is shown a suction plate 11 formed of porous material whose outer diameter is smaller than that of a wafer 10, and surrounded around the outer periphery thereof by an airtight material 11b. In this case, less grinding fluid is drawn into the vacuum suction device, and the clamping force is stable in comparison with the example shown in FIG. 1. However, the clamping force is reduced on the outer periphery of the wafer 10 which is not held by suction. This will reduce the grinding accuracy around the outer periphery of the wafer 10. In addition, since a material forming the suction plate 11 and a material forming the outer face 11b of the suction plate 11 are different, differences between them in thermal expansion and deformation by humidity sometimes distort the suction face 11a. Thus, this method gives a result similar to the above. And yet, when grinding the suction face 11a, the outer surface 11b formed of different material is ground simultaneously. This causes damage to a grinding wheel and a cutter.
Also, in the example shown in FIG. 2, another suction plate 11 is available whose outer periphery is formed of synthetic resin. In this case, there is the possibility that the resin comes up to the suction face 11a from the inner part of the porous suction plate 11, and as mentioned before, the suction face becomes uneven or deformed owing to the difference between both materials in expansion coefficient. In any case, no vacuum suction device has heretofore been available which is stable enough to permit holding workpieces such as a wafer by suction.